Calcium incorporation by smooth muscle microsomes

Area of Science:

• Cellular Physiology
• Muscle Biology
• Biochemistry

Background:

• Microsomal fractions are crucial for understanding intracellular calcium handling.
• Calcium ions play a vital role in smooth muscle contraction.
• Investigating calcium incorporation mechanisms is key to understanding muscle function.

Purpose of the Study:

• To identify factors influencing calcium incorporation into guinea-pig ileum smooth muscle microsomes.
• To differentiate between calcium binding and calcium uptake processes.
• To explore the role of oxalate and ion concentrations on calcium handling.

Main Methods:

• Preparation of microsomal fractions from guinea-pig ileum longitudinal smooth muscle.
• Measurement of calcium-45 (45Ca) incorporation and binding.
• Assays conducted with varying concentrations of ATP, Mg2+, oxalate, Ca2+, Na+, K+, and choline.
• pH-dependent studies and subcellular distribution analysis using marker enzymes.

Main Results:

• Calcium incorporation requires ATP and Mg2+, is enhanced by oxalate (optimal at pH 6.6), and has an optimal Ca2+ concentration of 3x10^-5 M.
• Two distinct Ca2+ binding sites were identified in the absence of oxalate: low affinity (Kd=7x10^4 M^-1) and high affinity.
• Oxalate-stimulated uptake and Ca2+ binding activities are both microsomal but exhibit different sedimentation rates, suggesting distinct structures.

Conclusions:

• Two separate processes likely mediate Ca2+ incorporation and binding in ileum microsomes.
• Oxalate enhances Ca2+ uptake, potentially involving a distinct mechanism from passive binding.
• These findings offer insights into microsomal calcium regulation and its potential role in excitation-contraction coupling.